A. F. van Herwaarden

'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser II.Carbohydrate and protein dynamics

Changes in carbohydrate and protein in stems, leaves, spikes, and grain between anthesis and maturity were measured in 3 dryland wheat crops whose responses to applied nitrogen (N) ranged from increases in grain yield through to decreases in grain yield. This decrease in grain yield, known as haying-off, was described in Paper I in this series. Measurements reported there showed that apparent retranslocation, defined as the decrease in weight of vegetative organs during grain filling, was generally greater for crops of high-N status than for those of low-N status. Retranslocation in this context is the process of moving compounds assimilated before anthesis to the grain. The largest source of assimilates available for retranslocation in all crops at anthesis was water-soluble carbohydrates (WSC) contained in the stems and spikes, and represented a potential contribution of 34-50% to yield for the most severely hayed-off crops. The absolute amount of WSC present in high-N crops was less than that in low-N crops, despite a greater biomass. The lack of this form of assimilate available for retranslocation was the greatest single contributor to the yield reduction of the crops of high-N status. The quantity of protein retranslocated increased with crop N status, but the amounts involved were smaller than the quantity of WSC. Virtually all of the WSC reserves were utilised in all crops, in contrast to the protein reserves which were poorly retranslocated in the hayed-off crops. Most of the WSC was contained in the stems and most of the protein in the leaves. The potential contribution of retranslocated WSC and protein from leaves was more difficult to estimate because of an apparent loss of 40-50% of leaf tissue after anthesis. The nature of the loss was estimated from the amounts of acid detergent fibre (ADF; fibre not solubilised by hot acid detergent) present at anthesis and maturity. Since ADF comprises cellulose and lignin which decompose slowly, the loss of 30-37% of ADF was applied as a correction factor in calculating potential retranslocation from leaves. There was no loss of stem ADF. Using the correction, the potential retranslocation of leaf protein and leaf WSC was equivalent to 6-15% of yield. The export of all WSC and protein failed to account for the total decrease in leaf biomass, even after correction of leaf losses. We identified hemicellulose as an additional and previously unsuspected source of carbohydrate for retranslocation. Unlike WSC, the amount of leaf and stem hemicellulose at anthesis increased with crop N status, and the increase in hemicellulose between anthesis and maturity was equal to 10-17% of yield.

Analysis of the benefits to wheat yield from assimilates stored prior to grain filling in a range of environments

Grain yields of rainfed agriculture in Australia are often low and vary substantially from season to season. Assimilates stored prior to grain filling have been identified as important contributors to grain yield in such environments, but quantifying their benefit has been hampered by inadequate methods and large seasonal variability. APSIM-Nwheat is a crop system simulation model, consisting of modules that incorporate aspects of soil water, nitrogen (N), crop residues, crop growth and development. Model outputs were compared with detailed measurements of N fertilizer experiments on loamy soils at three locations in southern New South Wales, Australia. The field measurements allowed the routine for remobilization of assimilates stored prior to grain filling in the model to be tested for the first time and simulations showed close agreement with observed data. Analysing system components indicated that with increasing yield, both the observed and simulated absolute amount of remobilization generally increased while the relative contribution to grain yield decreased. The simulated relative contribution of assimilates stored prior to grain filling to grain yield also decreased with increasing availability of water after anthesis. The model, linked to long-term historical weather records was used to analyse yield benefits from assimilates stored prior to grain filling under rainfed conditions at a range of locations in the main wheat growing areas of Australia. Simulation results highlighted that in each of these locations assimilates stored prior to grain filling often contributed a significant proportion to grain yield. The simulated contribution of assimilates stored prior to grain filling to grain yield can amount to several tonnes per hectare, however, it varied substantially from 5–90% of grain yield depending on seasonal rainfall amount and distribution, N supply, crop growth and seasonal water use. High N application often reduced the proportion of water available after anthesis and decreased the relative contribution of remobilization to grain yield as long as grain yields increased, particularly on soils with greater water-holding capacity. Increasing the capacity or potential to accumulate pre-grain filling assimilates for later remobilization by 20% increased yields by a maximum of 12% in moderate seasons with terminal droughts, but had little effect in poor or very good seasons in which factors that affect the amount of carbohydrates stored rather than the storage capacity itself appeared to limit grain yield. These factors were, little growth due to water or N deficit in the weeks prior to and shortly after anthesis (when most of the assimilates accumulate for later remobilization), poor sink demand of grains due to low grain number as a result of little pre-anthesis growth or high photosynthetic rate during grain filling. Increasing the potential storage capacity for remobilization is expected to increase grain yield especially under conditions of terminal drought.

Quantitative trait loci for water-soluble carbohydrates and associations with agronomic traits in wheat

Several environmental factors including drought and disease can reduce leaf area and photosynthesis during grain-filling to decrease grain yield and kernel weight of cereal crops. Water-soluble carbohydrates (WSC) accumulated around anthesis can be mobilised to assist in filling of developing grains when post-anthesis assimilation is low. Cultivar differences support opportunities to select for high WSC but little is known of the extent or nature of genetic control for this trait in wheat. Three wheat mapping populations (Cranbrook/Halberd, Sunco/Tasman, and CD87/Katepwa) were phenotyped for WSC and other agronomic traits across multiple environments. The range for WSC concentration (WSC-C) was large among progeny contributing to moderate-to-high narrow-sense heritabilities within environments (h2 = 0.51–0.77). Modest genotype × environment interaction reduced the correlation of genotype means across environments (rp = 0.37–0.78, P < 0.01) to reduce heritability on a line-mean (h2 = 0.55–0.87) basis. Transgressive segregation was large and genetic control complex, with 7–16 QTLs being identified for WSC-C in each population. Heritability was smaller (h2 = 0.32–0.54) for WSC mass per unit area (WSC-A), reflecting large genotype × environment interaction and residual variance with estimating anthesis biomass. Fewer significant QTLs (4–8) were identified for this trait in each population, while sizes of individual genetic effects varied between populations but were repeatable across environments. Several genomic regions were common across populations including those associated with plant height (e.g. Rht-B1) and/or anthesis date (e.g. Ppd1). Genotypes with high WSC-C were commonly shorter, flowered earlier, and produced significantly (P < 0.01) fewer tillers than those of low WSC-C. This resulted in similar yields, lower final biomass, and fewer grains per m2, but greater dry weight partitioning to grain, kernel weight, and less grain screenings in high compared with low WSC-C genotypes. By contrast, lines high for WSC-A produced more fertile tillers associated with similar or greater anthesis and maturity biomass, grain number, and yield, yet similar kernel weight or size compared with genotypes with low WSC-A. The data support an important role for WSC-A in assuring stable yield and grain size. However, the small effects of many independent WSC QTLs may limit their direct use for marker-aided selection in breeding programs. We suggest using molecular markers to enrich populations for favourable height and anthesis date alleles before the more costly phenotypic selection among partially inbred families for greater WSC-A.

Soil water extraction by dryland crops, annual pastures, and lucerne in south-eastern Australia

The extraction of soil water by dryland crops and pastures in south-eastern Australia was examined in 3 studies. The first was a review of 13 published measurements of soil water-use under wheat at several locations in southern New South Wales. Of these, 8 showed significantly more water extracted by crops managed with increased nitrogen supply or growing after a break crop. The mean additional soil water extraction in response to break crops was 31 mm and to additional N was 11 mm. The second study used the SIMTAG model to simulate growth and water-use by wheat in relation to crop management at Wagga Wagga. The model was set up to simulate crops that produced either average district yields or the potential yields achievable with good management. When simulated over 50 years of weather data, the combined water loss as drainage and runoff was predicted to be 67 mm/year for poorly managed crops and 37 mm for well-managed crops. Water outflow was concentrated in 70% of years for the poorly managed crops and 56% for the well-managed crops. In those years the mean losses were estimated to be 95 mm and 66 mm, respectively. The third study reports soil water measured twice each year during a phased pasture-crop sequence over 6.5 years at Junee. Mean water content of the top 2.0 m of soil under a lucerne pasture averaged 211 mm less than under a subterranean clover-based annual pasture and 101 mm less than under well-managed crops. Collectively, these results suggest that lucerne pastures and improved crop management can result in greater use of rainfall than the previous farming systems based on annual pastures, fallows, and poorly managed crops. The tactical use of lucerne-based pastures in sequence with well-managed crops can help the dewatering of the soil and reduce or eliminate the risk of groundwater recharge. Additional keywords: water-use, groundwater recharge, simulation, dryland salinity, soil dewatering. J.s, R.lt, les, per,n Herwa AR3 Watersd p J.us et a

Agronomic evaluation of a tiller inhibition gene (tin) in wheat. I. Effect on yield, yield components, and grain protein

Reduced tillering cereals have been proposed as being advantageous under terminal drought conditions through their reported reduction in non-productive tiller number and reduced soil water use prior to anthesis. This study was conducted to determine whether wheat (Triticum aestivum L.) lines containing the tiller inhibition (tin) gene have a yield penalty over their commercial near-isogenic counterparts. A terminal drought was experienced in all experiments. The effects of the tin gene were investigated in 4 different near-isogenic pairs of lines grown at 2 sowing densities at 4 locations in the eastern Australian wheatbelt over a 3-year period. Averaged over all experiments and lines, grain yield was unaffected by the presence of the tin gene. However, the highest yielding line contained the tin gene and its yield was 5% higher than all other lines. Averaged across the different genetic backgrounds, the tin gene decreased fertile spike number by 11%, increased the number of kernels/spike by 9%, and there was a 2% increase in kernel weight. The tin gene increased the harvest index by an average of 0.02, whereas above-ground biomass was reduced by 7%. Increasing sowing density from 50 to 100 kg/ha had little influence on yield or yield-related characteristics in both the restricted tillering and freely tillering lines. There was an interaction between sowing rate and the presence of the tin gene on yield, with tin lines yielding 0.2 t/ha more than the freely tillering lines at the higher sowing rate, whereas there was no effect at the lower sowing rate. The response of several lines containing the tin gene to nitrogen fertiliser was also investigated at 2 sites. Nitrogen increased spike number in all lines but the number remained around 20% less than in the freely tillering cultivars. The yield of wheat lines containing the tin gene was 6% greater than their near-isogenic pairs where nitrogen status was high in the presence of terminal drought. Grain protein concentration was unaffected by the presence of the tin gene at high grain protein sites, whereas at lower grain protein sites it had a positive effect.

Simulation of grain protein content with APSIM-Nwheat

Abstract apsim -Nwheat is a wheat crop system simulation model within the apsim framework which consists of modules that incorporate aspects of soil water, nitrogen, residues, wheat ( Triticum aestivum L.) crop development and growth, including grain protein content. apsim -Nwheat has been validated for soil water, soil N, crop phenology, biomass production and yield. However, previous analyses have indicated that model performance was poor in terms of grain protein simulations particularly under very high or very low N input conditions together with terminal drought. The original routine for grain protein content simulated grain protein as a function of independent dry matter and N accumulation into the grain. To constrain grain protein content simulations under very high and very low N input conditions, without effecting other simulations, a simple but physiologically sound link was incorporated between dry matter and N translocation to the grain. An upper boundary of daily protein transfer to the grain was set to 4% N (22.8% grain protein), and a lower boundary was set at 1.23% N (7% grain protein). In addition, grain N was initialised with up to 3% N (17.1% grain protein) at the beginning of the grain filling phase, depending on the availability of tissue N. The new modified grain protein routine was tested across field data sets from a wide range of growing conditions and showed an improved performance with a RMSD, consistent across environments and soil types, at or below 0.35% N (2% grain protein). Independent tests, including data from different climates and a sensitivity analysis confirmed the improved simulation of grain protein under a wide range of conditions. The improved model was found to be reliable and robust enough to be used for specific simulation experiments to study grain protein interactions with management, soil types and environments in different climatic regions.

Variation for and relationships among biomass and grain yield component traits conferring improved yield and grain weight in an elite wheat population grown in variable yield environments

Grain yield and kernel size (grain weight) are important industry traits for wheat in the water-limited environments of the north-eastern wheatbelt of Australia. These, and underpinning morphological and physiological traits, were evaluated in a population of recombinant inbred lines from the elite CIMMYT cross Seri/Babax, segregating for the presence of the rye translocation (T1BL.1RS). The population was examined to determine the variation among lines, relationships among traits, the extent of line × environment interactions, potential efficiency of direct and indirect selection, and to identify trait combinations that are associated with higher grain yield and grain weight. Transgressive segregation was observed for all traits, and line × environment interaction effects were frequently larger than line main effects. Across six environments ranging in yield from 202 to 660 g/m2, the T1BL.1RS wheat-rye translocation had a positive effect on grain weight (+3.4%) but resulted in decreased grain number per m2 (–6.5%) and grain yield (–3.1%). Realised selection responses indicated that broad adaptation was best achieved by selection for mean performance across the range of target environments. However, specific adaptation for performance in high- or low-yielding environments was best detected by direct selection in those types of environments. A group of broadly adapted Seri/Babax lines exceeded the mean of five cultivars grown commercially in the north-eastern wheatbelt by 8% for grain yield and 17% for grain weight. These Seri/Babax lines with both high grain yield and grain weight were associated with a combination of several traits: earlier flowering, reduced tillering, a greater proportion of tillers that produce grain-bearing spikes at maturity, high water-soluble carbohydrate stem reserves at anthesis, a higher proportion of competent florets at anthesis to maximise grains per spikelet leading to a high harvest index, and possibly a greater capacity to extract soil water. These results suggest a suitable ideotype for breeding high-yielding wheat cultivars with high grain weight adapted to environments with hotter, drier conditions during the post-anthesis period.

Agronomic evaluation of a tiller inhibition gene (tin) in wheat. II. Growth and partitioning of assimilate

Wheats with reduced tillering have been proposed for areas regularly subject to a terminal drought. A wheat plant with a genetic disposition to produce fewer stems is now possible through the introgression of a gene that inhibits tillering (tin). This study was conducted to determine the effect of the tin gene on the dynamics of tillering, light interception, and dry-matter production and partitioning in several different cultivars of wheat. Commercial cultivars and their near-isogenic pairs differing in the presence of the tin gene were grown in well-watered tubes and also in the field in south-eastern Australia where terminal drought is common. Tiller number, light interception, leaf area index (LAI), biomass, and the partitioning of biomass were recorded at various intervals throughout the growing season. Water-soluble carbohydrate (WSC) levels in the stems of field-grown plants were also determined in some environments at anthesis and maturity. In tubes and in field environments, lines with the tin gene produced tillers at the same rate as their free tillering counterparts but ceased tillering sooner. Under conditions where the free tillering lines produced over 1000 shoots/m2, lines containing the tin gene produced 600 shoots/m2. However, by maturity, fertile spike numbers were 350 and 450/m2 for lines with and without the tin gene, respectively. Despite the large difference in tillering, there were only small differences in LAI, light interception throughout the season, and biomass. There were small differences in the proportional allocation of biomass, and the tin lines partitioned more of their biomass towards spikes at anthesis and stored more WSC in stems. Dry weight distribution varied with genetic background, but in general the tin gene increased leaf area ratio and root to shoot ratio but decreased specific leaf area. It is concluded that the tin gene may be advantageous under terminal drought. This would come from the reduced light interception prior to anthesis and thereby potential for greater transpiration during grain filling as well as a greater capacity for stem carbohydrate storage and remobilisation. These factors are consistent with a greater harvest index and kernel weight associated with lines containing the tin gene.

Genetic Control of Water-Soluble Carbohydrate Reserves in Bread Wheat

The combination of high temperatures and lack of water decrease leaf area and reduce carbon assimilation in a terminal drought. It has been suggested that selection for greater stem storage of water soluble carbohydrate (WSC) would result in improved grain-filling and increased yields in drought-prone environments. No study has reported the extent or nature of genotypic variation for WSC in wheat. Progeny from three populations were genotyped with between 450 and 950 polymorphic molecular markers and phenotyped for stem WSC in well-watered environments. The range of WSC among progeny was large contributing to moderate to high narrow-sense heritabilities within environments ( h2=0.58 to 0.77); while relatively small genotype × environment interactions increased heritability on a genotype-mean basis (0.67 to 0.83). Large transgressive segregation suggested that stem WSC is controlled by multiple loci. Genetic control was complex with between 7 and 10 significant QTL (e.g. chromosomes 1A, 1B, 2B, 2D, 4B, 7A and 7B) identified for WSC across populations and environments. QTL were generally of small effect, each accounting for between 3 and 28% of the genotypic variance. Progeny with high WSC produced higher grain weight and larger diameter, significantly reducing grain shrivelling. High heritability indicates potential for phenotypic selection of WSC among families in breeding programs that target adaptation to terminal droughts